Electron multipliers have been developed in a variety of forms over the years. In its simplest form it can be made as a single channel device in a vacuum envelope, having an input and output electrode to produce a linear accelerating electric field, a generally tubular channel member, the inner surface of which is capable of a secondary electron yield greater than 1. Another essential property of the channel is a fair conductivity which insures that electrons removed from the walls by secondary emission are promptly replaced but which is low enough to support the required electric field along the channel without drawing excessive current.
Other factors which have received considerable attention are the angle of the walls relative to the electric field lines and the problem of positive ion generation due mostly to impurities in the tube. These factors are interrelated fortunately, so that increasing the angle both increases the probability of the electron hitting the tube and decreases the probability of the positive ion hitting the cathode before striking the tube. Periodically reversing the angle of bend has also been employed to enhance these probabilities. These techniques are fairly simple with a single channel device, but became rather complicated when numerous channels with fixed geometric relationships between the inputs and outputs are involved. Such a device is the microchannel plate (see U.S. Pat. Nos. 3,497,759 and 3,528,101 granted Feb. 24, 1970 and Sept. 8, 1970 to B. W. Manley).
The microchannel plate is formed from a plurality of glass pipes or hollow fibers which are heated as a bundle and drawn to microscopic diameters. A limited amount of twisting can accompany the drawing operation to achieve angular relationships as discussed above, but this cannot be allowed to disturb significantly the relative positions of the fibers or their cross-sectional shape. Such twisting probably would be sufficient if the fibers were made from a material having a very high coefficient of secondary emission. Unfortunately the best glass from the standpoints of ease of fabrication and cost have rather low coefficients. Thus, added to uncertainty of when the first collision between the wall and the electron will occur, is the uncertainty that a significant number of secondary electrons will be emitted whenever such a collision occurs. The electron causing the avalanche may itself be initiated by secondary electron emission, ion induced electron emission, photo-emission, or other initiating event capable of liberating an electron into a vacuum.
The function of a typical electron-multiplier device is that of producing a large number of electrons at the output on receipt of an initiating event. In order that the device may be an efficient detector, the probability of not responding to an event must be small. Further, if the device is to be used as a linear amplifier, the size of the output pulse must be uniform. A measure of the spread in pulse sizes is the resolution of the device. A device with high resolution contributes little noise to the signal.
These two attributes of a multiplier, detection efficiency and resolution, combine to produce a signal such that the contrast between a weak and a strong source is reduced. In an image tube, this results in a reduction in the range at which an object can be seen, at constant light level. In a pulse detector it means a loss in amplitude discrimination between pulses. In a pulse counter, a low detection efficiency results in some fraction of input pulses not being detected at all.